The background art is described by taking a shovel machine as an example.
In a normal shovel which has only an engine as a power source, horsepower control is carried out to control a flow rate of a hydraulic pump in accordance with a load pressure of a hydraulic pump (discharge power of a hydraulic pump). The horsepower control is carried out in accordance with the characteristics shown in FIG. 3. More specifically, the characteristics shown in FIG. 3 are set in such a manner that the flow rate is a maximum at control starting pressure (the pressure division A in FIG. 3) and the flow rate is a minimum at maximum pressure (relief pressure).
In the characteristics shown in FIG. 3, the maximum input setting of the hydraulic pump is set to be no greater than the maximum output of the engine, in such a manner that the engine does not become overloaded. More specifically, the maximum input setting of the hydraulic pump is set so as to be smaller than the maximum output of the engine by a substantially constant value.
More specifically, with the characteristics shown in FIG. 3, the following relationship is established.Maximum output of engine>Maximum input of hydraulic pump=Discharge pressure of hydraulic pump×flow rate
(Note that the Efficiency and the Coefficient are Omitted Here)
Generally, a combination of flow rate control and the horsepower control described above is used to control the flow rate of the hydraulic pump. More specifically, the lower flow rate is selected, of the flow rates calculated respectively by flow rate control and horsepower control, and the selected flow rate is instructed to the regulator of the hydraulic pump. The flow rate control described above is a control system for controlling the flow rate of the hydraulic pump in accordance with the amount of operation of operating means which operates a hydraulic actuator (called “lever operation amount” below).
In the flow rate control described above, the flow rate is set to a standby flow rate (a flow rate which is determined by taking account of the system responsiveness at the start of operation) when the operating means is in a neutral state, whereas the flow rate is set to a maximum flow rate when the operating means is fully operated.
On the other hand, in the horsepower control described above, the flow rate is set to a maximum flow rate when at the control starting pressure, whereas the flow rate is set to a minimum flow rate when at the relief pressure.
Consequently, in the pressure division A described above, a flow rate for the hydraulic pump is instructed on the basis of flow rate control in accordance with the amount of operation of the operating means, and in the pressure range of the hydraulic pump exceeding this division A, a flow rate for the hydraulic pump is instructed on the basis of horsepower control.
On the other hand, the hybrid shovel includes a hydraulic pump which drives a hydraulic actuator, a generator-motor which can operate as an electric generator and as an electric motor, and an engine to which the hydraulic pump and the generator-motor are connected. In this hybrid shovel, an electric storage device is charged by operation of the generator-motor as an electric generator, and driving of the hydraulic pump is assisted by operation of the generator-motor as an electric motor, using electric power discharged from the electric storage device.
The flow rate characteristics of the hydraulic pump in the hybrid shovel are basically set to be the same as a normal shovel, in order to maintain the mechanical characteristics of the normal shovel.
However, the assistance provided by the generator-motor (electric storage device) as described above is added to the power of the engine in the hybrid shovel. Consequently, as shown in FIG. 4, the maximum output of the engine in the hybrid shovel is set to a value that is lower than the maximum output of the engine in a normal shovel, taking account of the assistance described above (normally, an average power; this applies to the following description as well).
More specifically, with the characteristics shown in FIG. 4, the following relationship is established.Maximum output of engine<Maximum input of hydraulic pump
Moreover, with the characteristics shown in FIG. 4, the following relationship is also established.(Maximum output of engine+Maximum assistance power)>Maximum input of hydraulic pump
This hybrid shovel has the following two intrinsic problems. Firstly, when the amount of charge of the electric storage device has declined, the assistance capability described above declines, and when the remaining amount of charge of the electric storage device falls below a limit, the assistance capability is lost. Therefore, the engine becomes overloaded and there is a risk of stalling.
Secondly, with frequent high-level charging and discharging, severe deterioration of the electric storage device occurs.
As a countermeasure for problems of this kind, technology has been proposed for limiting the maximum input of the hydraulic pump in accordance with the amount of charge in the electric storage device (see, for example, Patent Document 1).
The control described in Patent Document 1 is control based on a supportive approach in that the progress of discharging and the occurrence of sudden charging and discharging are suppressed by limiting the maximum input of the hydraulic pump in accordance with decline in the amount of charge in the electric storage device. Consequently, during normal work where the wear (discharging) of the electric storage device is gradual, the control described in Patent Document 1 is effective.
However, under high-load pressure conditions (and in particular, relief pressure conditions) such as a warm-up operation immediately after starting up the engine in cold conditions, or rock excavation work, if the hydraulic pump is operated continuously, in other words, if the hydraulic pump is continuously operated at full power, then the electric storage device wears (discharges) rapidly at a high level. In circumstances such as these, even if the control described in Patent Document 1 is implemented, wear of the electric storage device cannot be suppressed and the assistance capability of the electric storage device declines suddenly. Consequently, during subsequent normal work, the assistance capability becomes insufficient or it becomes impossible to provide assistance, thus creating a hindrance to work tasks.    Patent Document 1: Japanese Unexamined Patent Publication No. 2005-83242